Real number - A number which can be represented uniquely by a point on axis.

And real number = rational + irrational number

By this definition, if you take an irrational number such as sqrt(2) ...it's value is non terminating non repeating (1.414....)

Doesnt this violates the def of real number? Since by this way the exact location of the point( sqrt(2) ) can never be identified.

Note: I know that sqrt(2) can be represented on axis using Pythagoras theorem and making a perpendicular of unit length on unit length on x axis. By this way also I can uniquely plot sqrt(2) on axis

But then again this would mean that the decimal expansion of sqrt(2) should terminate at a value.

Any ideas?


  • $\begingroup$ While the exact location may never be identified, it is still a unique point on a number line. $\endgroup$ – Christopher Marley May 2 '18 at 4:46
  • $\begingroup$ ""Doesnt this violates the def of real number? Since by this way the exact location of the point( sqrt(2) ) can never be identified." No. Just because we can't place it in terms of decimals does not mean we can place it in terms of $\sqrt{2}$. The square root of two is the length of a diagonal of a unit square. That is exact. Being able to identify it by it's decimal is not important. All real numbers and all points on a line are unique. Or not being able to identify them does not make them not exist. The real numbers are all points on a line whether we can identify them or not $\endgroup$ – fleablood May 2 '18 at 4:46
  • $\begingroup$ "But then again this would mean that the decimal expansion of sqrt(2) should terminate at a value." No. Because decimals are not important. They are not the end all or be all of anything. They are only a way of approxiamately with arbitrary (but not absolute) precision. And they are only one way of many of doing that. $\endgroup$ – fleablood May 2 '18 at 4:51
  • 1
    $\begingroup$ I will point out as well that this definition of real numbers is hardly rigorous since the real number line hasn't yet been defined. Look up a more rigorous definition using dedekind cuts or cauchy sequences. $\endgroup$ – JMoravitz May 2 '18 at 5:17
  • $\begingroup$ @fleablood thanks for your reply $\endgroup$ – lancer May 2 '18 at 14:06

I think you're confusing "represented" and "located". Yes, all real numbers can be represented by the points on a line. But remember that a line is an abstract thing. You can never draw or imagine a line (only represent it approximately). Once you realise these things you should be more clear about this (admittedly deep matter -- check out "continuum").

So, yes, whereas every point on a straight line can be made to correspond to one and only one real number, it does not mean we can always locate this point for all real numbers (by locate I mean using a finite sequence of steps). You may call this theology, but math sometimes often deals in such things, especially in relation to deep matters like this.

The real numbers we can locate are known as constructible numbers. But perhaps even if we allow ourselves more freedom, we may even find rectilinear lengths of any real size and use it to "locate" our points.

But don't think all is settled, or that you can finish settling this in a few hours. It's a very interesting matter that one keeps getting back to.

So how should one think rigorously of real numbers (wlog irrational numbers)? Well, they were defined because extracting the roots and logarithms of rationals does not always give a rational value. But since we know these things must correspond to some quantity, we call them irrationals (this is not very different from the invention of negative numbers and complex numbers). The special thing about irrational numbers is that the involve the "cumulation" of infinitely many operations, loosely speaking. Dedekind was first to give a rigorous definition in terms of order relations and sets (so-called Dedekind cuts).

  • $\begingroup$ Thanks .. I got your point. I would request you to give your inputs of the answer given by me above. $\endgroup$ – lancer May 5 '18 at 13:28

One way to think about this:

Whenever we mention a number on a real number line, we technically are dividing the number line into equal parts and mentioning the value.

For example say a number 10, it means dividing the whole line into equal parts starting from the origin, and the 10th point is your number.

Similarly when we specify the point 0.35 what we actually are doing is that we are further subdividing the number line between 0 and 1, into 20 equal parts, and the 7th part is 0.35 ( 7/20). In a very similar way we are representing 0.3 ( which is 3/10...by dividing the number between 0 to 1 into 10 equal parts and the third point is 3/10)..

In a very similar way 2.4 etc etc.

But when we represent sqrt2, etc, we are able to locate the point on a number line ( using Pythagoras theorem). But there is no such whole number of partitions possible between 1 and 2, out of which one point of that partition will be sqrt(2)... Hence the decimal representation goes on till infinity and never terminates nor repeats.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.